Now when user subscribe, they get to choose 1 "OR" more categories.
I would like to allow user to subscribe "AND" categories, which means, when a post is posted in all the categories selected by user, then only will subscriber get notified.

My blog has posts posted in multiple categories, so i would like a subscription and will notify subscriber only when a post is posted in certain categories.

This is possible as it's just a database querry, but the problem is that a intuitive user interface is needed. When designing the current one, I had lots of Ideas in that direction, but discarded them all... If you know php have a look at the sendmail.php:post_notification_send() function.

I'm not a php programmer, but is willing to learn and try to mod it. I'll google it and ask for help.

about the user interface, which file that i need to modify on? for example, instead of tick box, i could change it to with several drop down menus, specifying only limited categories for each drop down menu, and "AND" them together for subscription.